1. Field of the Invention
The present invention relates to the field of electrical circuits. More particularly, the invention relates to amplifier circuits.
2. Background Art
An amplifier, such as low-noise amplifier (LNA) used in cellular handsets, can be required to meet specific linearity and enable time requirements. The linearity of an amplifier, such as an LNA, is typically determined at a particular frequency and can be characterized as a function of input power. For example, the linearity of an amplifier, such as an LNA, can be specified in terms of input third-order intermodulation intercept point (IIP3) performance at a particular tone spacing frequency, which refers to the spacing, i.e., difference in frequency, between two test tones that are used to determine the value of IIP3. The enable time, i.e., turn-on time, of an amplifier, such as an LNA, corresponds to the amount of time required to achieve a final, i.e., steady-state, bias voltage at the bias input of the amplifier.
In a conventional amplifier, such as an LNA, a tank circuit can be utilized at the output of the bias circuit to provide a low impedance at a particular tone spacing frequency so as to meet a linearity specification for IIP3 at that tone spacing frequency. However, for a low tone spacing frequency, such as 15.0 kHz, a large capacitance is typically required in the tank circuit to provide the low impedance necessary to meet IIP3 requirements, which can undesirably increase the enable time of the amplifier by increasing the RC time constant at the bias input of the amplifier. Thus, it is difficult for a conventional amplifier, such as a conventional LNA, to meet IIP3 specifications at low tone spacing frequencies, such as 15.0 kHz, and also meet a fast enable, i.e., turn-on, time specification.